In the field of mass analysis using an ion trap mass spectrometer or other apparatuses, a technique called the MS/MS analysis (or tandem analysis) is conventionally known. In a general MS/MS analysis, an ion having a specific mass number (mass-to-charge ratio m/z) is first selected as a precursor ion from an object to be analyzed. Next, the precursor ion thus selected is dissociated by a collision induced dissociation (CID) process to produce product ions. After that, the product ions are mass analyzed to obtain the information on the mass-to-charge ratio of the product ions and desorbed ions, and based on this information, the composition and chemical structure of the target sample molecule are deduced.
In recent years, samples having larger molecular weight than before have been analyzed with such an apparatus, and their chemical structures (composition) have also tended to become more complicated. Hence, depending on the quality of the sample, ions are often not dissociated to have a sufficiently small mass by only a one-stage dissociation operation. In such cases, an MSn analysis may be performed in which a dissociation operation is repeated multiple times and the product ions finally generated are mass analyzed (refer to Patent Documents 1 and 2 for example). The aforementioned MS/MS analysis is an MSn analysis in the case where n=2.
Generally, in mass spectrometers, a mass spectrum (MSn spectrum) in which a mass-to-charge ratio is assigned to the horizontal axis and a signal intensity (or relative intensity) to the vertical axis is created as a result of a mass analysis, and the mass spectrum is displayed on a display screen as one of the analysis results. In mass spectrometers capable of an MSn analysis, if a plurality of precursor ions having different mass-to-charge ratios are set as candidates to be selected, each MSn spectrum for every precursor ion is obtained.
At this point, as an example, assume that the ions of m/z500 and m/z800 were selected as a precursor ion and an MS/MS analysis was performed for each of them, with the result that MS/MS spectra as illustrated in FIG. 9 were obtained. The position of each peak appearing on these MS/MS spectra indicates the mass-to-charge ratio of the product ions generated by the dissociation process. According to FIG. 9(a), it is understood that three kinds of product ions of m/z180, 240, and 400 are obtained in the case where m/z500 is set to be a precursor ion. According to FIG. 9(b), it is understood that two kinds of product ions of m/z480 and 700 are obtained in the case where m/z800 is set to be a precursor ion. In the cases for these examples, since almost all the precursor ions are dissociated by a dissociation operation, the peak corresponding to the precursor ion does not appear in the MS/MS spectra.
In an MSn analysis, when the precursor ion's chemical structure and composition are deduced and then the original sample molecule's structure and composition are deduced, the information about the mass-to-charge ratio of each product ion is important. Simultaneously, the mass information of the fragments removed in the process in which each product ion is generated is also important. However, even if a person in charge of an analysis looks at an MSn spectrum displayed as previously described in conventional apparatuses, it is not easy to intuitively obtain the information about the mass of the fragments removed in such a dissociation process.
In particular, in the case where the peak corresponding to a precursor ion clearly appears on an MSn spectrum, it is easy to mentally calculate the difference between the mass-to-charge ratio of the precursor ion and that of each product ion, i.e. the information of the mass of the removed fragments. However, in the case where the peak originating from the precursor ion can be hardly seen as in the example illustrated in FIG. 9, it is difficult to mentally perform the calculation as just described. In addition, in an MSn analysis, another important piece of information is whether or not there are desorbed fragments having the same mass for the dissociations of a plurality of precursor ions whose mass-to-charge ratios are different. However, it is also difficult to intuitively determine such a fact with an MSn spectrum displayed in conventional apparatuses as described earlier. Such inconveniences contribute to decreasing the working efficiency of the structure deduction based on the result of an MSn analysis.    [Patent document 1] Japanese Unexamined Patent Application Publication No. H10-142196    [Patent document 2] Japanese Unexamined Patent Application Publication No.